The invention relates to curable filled underfill bonding compositions. In the electronics industry, electronic components such as resistors, capacitors, inductors, transistors, integrated circuits, chip carriers, and the like are typically mounted on substrates or circuit boards in one of two ways. In the first way, the components are mounted on one side of the board and leads from the components extend through holes in the board. Solder is applied to the opposite side of the board to anchor the leads and provide an electrical connection. In the second way, the components are soldered to the same sides of the board upon which they are mounted. These latter devices are said to be “surface mounted”.
Surface mounting of electronic components is a desirable technique in that it may be used to make very small circuit structures and that it can easily be used in automated processes. One family of surface-mounted devices comprises integrated electronic components having numerous connecting leads attached to pads located on the surface of a substrate. Either the electronic component or the substrate to which it is attached is provided with small bumps or balls of solder positioned in locations that correspond to the bonding pads on the electronic component or the substrate. The electronic component is electrically mounted to the substrate by placing it in contact with the substrate such that solder bumps contact the corresponding pads on the substrate and the component. The assembly is heated to a point at which the solder melts or reflows and then the assembly is cooled so to bond the electronic component to the substrate through the solidified solder.
One problem with this technology is that the electronic component, the solder, and the substrate often have significantly different coefficients of thermal expansion. As a result, differing expansion rates as the assembly heats during use can cause severe stresses, for example, thermomechanical fatigue at the connections and lead to failures that impact the reliability of the device.
Thermoset epoxy resins have been used to minimize the effects of different thermal expansions. Polyepoxides have been used as an underfill that surrounds the periphery of the electronic component and occupies the space between the electronic component and the substrate that is not occupied by solder. Improved underfill materials have been formulated by incorporating conventional silica filler, that is, silica having a particle size of one micrometer to 50 micrometers or greater into the underfill composition. Although high loading levels of such silica filler may provide desirable coefficients of thermal expansion, such levels also may provide undesirable viscosities inhibiting the flow of the underfill bonding composition. Such particles may also get trapped around small features and in between small gaps when used to underfill electronic assemblies and may interfere with the electrical connection between the solder bumps and the bonding pads if the underfill bonding composition is placed between the component and substrate before the solder is reflowed. These larger filler particles also have a tendency to settle in underfill formulations. Settling of the filler particles prior to cure can result in an underfill having a non-uniform coefficient of thermal expansion. Settling of particles shortens shelf life and can also cause dispensing problems should this settling occur in the packaged underfill bonding composition.